Electronic circuits often use clock signals to regulate and control their operation. Events in the electronic circuits are timed by rising and/or falling edges of the clock signals. One or more main clock signals are typically provided in an electronic circuit, running at a particular clock speed or frequency. When clock signals having other frequencies are needed in the electronic circuit, they may be generated, for example, by a frequency synthesizer based on the main clock signal.
A typical frequency synthesizer may contain an oscillator such as an LC oscillator, having a tank circuit with an inductor and a capacitor. Energy is alternately transferred between the inductor and capacitor in oscillating fashion. The oscillating energy in the tank circuit is maintained by a current source that is alternately applied in different directions through the tank circuit under the control of cross-coupled transistors which form an effective negative resistance across the tank circuit. The output frequency of the frequency synthesizer is based on the resonant frequency of the tank circuit. It is generally very important that the output frequency be as accurate as possible. Although the output frequency of a frequency synthesizer is constantly corrected and adjusted by a feedback loop, electrical noise in the frequency synthesizer can cause variations in the instantaneous output frequency.
For example, flicker noise in an LC oscillator may adversely affect the oscillator phase noise. Flicker noise is an unwanted variation in the electrical signals in the LC oscillator originating, for example, in the cross-coupled transistors or transistors in the current source. Flicker noise may result from impurities in the gate oxide of MOS transistors, etc. Flicker noise over a large number of transistors has a distribution with a long tail, that is, there is typically a small percentage of the transistors in a group that are several dB worse than the average device. It is therefore not efficient to design a frequency synthesizer around the worst anticipated flicker noise in a particular type of transistor, because the large majority of the transistors in the group are much better than the worst. Flicker noise may also vary randomly with respect to temperature, making it difficult to compensate for flicker noise due to temperature variations in an oscillator during operation.